Color image sensors comprise a mosaic of individual filters covering respective sensor pixels. The filters may be red, green and blue (RGB), or alternatively cyan, magenta, yellow and green (CMYG). After an image has been formed on the sensor, signals from adjacent pixels are combined so as to produce local color and intensity values for the image. Although high-quality video cameras use three sensors—one for each primary color—mosaic sensors are used in almost all mass-market video devices because of their low cost.
FIG. 1 is a schematic diagram of a “Bayer-type” mosaic color imaging sensor, as is known in the art. A sensor of this type is the VV6500, produced by STMicroelectronics of Carrollton, Dallas, Tex. Color filters in the sensor are positioned on a rectangular grid, there being twice as many G filters as R and B filters. To generate an image of a region 12, signals from four pixels 14 comprising the region are combined to form a color signal having a luminance Y and a chrominance C of the region. Values of Y and C are calculated as functions, typically linear functions, of the signals from the individual pixels 14, i.e.,
 Y=F1(R, B, G1, G2), andC=F2(R, B, G1, G2),   (1)wherein R, B, G1, and G2 correspond to signals from their respective pixels, and F1 and F2 are functions.
It will be appreciated that the resolution of a color imaging sensor is less than that of a black and white imaging sensor with the same pixel pitch, since the color sensor averages adjacent pixels.
FIGS. 2 and 3 are schematic diagrams showing passage of light rays through a lens, as is known in the art. FIG. 2 shows white light rays 16, parallel to an axis 20 of a lens 18, incident on the lens. Because of dispersion by the lens, which dispersion is an inherent characteristic of all practical lens systems, the parallel rays are refracted to different foci on axis 20, according to the wavelength, i.e., the color, of the dispersed light. Thus, a blue focus 22 is closer to lens 18 than a red focus 24, and a green focus 26 is intermediate between the red and blue foci. Chromatic distortions caused because the red, blue, and green foci do not coincide on the lens axis are termed axial color aberrations.
FIG. 3 shows a white ray 28, i.e., a ray that exits lens 14 (in the absence of aberrations) substantially parallel to axis 20, and which thus defines a height of an image produced at an image plane 30. As for parallel rays 16, ray 28 is dispersed into its constituent colors, so causing a chromatic distortion termed lateral color aberration at the image plane.
Other distortions when a lens forms an image are also known in the art. For example, a square object may be imaged with “barrel” or “pincushion” distortion. Also, each point on the object will typically be imaged, according to a point spread function depending on the lens, to a more or less blurred region of the image having an area larger than zero. Methods for correcting distortions of the types described above, which are typically not functions of the wavelength of the imaging light, are known in the art. Examples of methods for deblurring images are described in an article titled “Review of image-blur models in a photographic system using the principles of optics” by Hsien-Che Lee, in the May, 1990 issue of Optical Engineering, which is incorporated herein by reference. Examples of distortion correction are described in an article titled “Digital Image Restoration” by Banham et al., in the March 1997 issue of IEEE Signal Processing Magazine, which is incorporated herein by reference. The book titled “Digital Image Restoration” by H. C. Andrews and B. R. Hunt, published by Prentice-Hall of Englewood Cliffs, N.J. in 1977, describes general methods for restoration of distorted images.